fix(server): fix leak in FuturesUnordered (#1204)

* fix: remove needless clone in ws background task

* fix(server): fix leak in FuturesUnordered

The tokio::spawn handles were never removed from `FutursUnordered`
which this commit fixes.

Reduces the memory usage signficantly but still slightly worse than v0.16.x

* Update server/src/transport/ws.rs

* cargo fmt

* wording